Extracellular proteoglycans modify TGF-β bio-availability attenuating its signaling during skeletal muscle differentiation

The onset and progression of skeletal muscle regeneration are controlled by a complex set of interactions between muscle precursor cells and their environment. Satellite cells constitute the main source of muscle precursor cells for growth and repair. After skeletal muscle injury, cell-derived signals induce their re-entry into the cell cycle and their migration into the damaged zone, where they proliferate and differentiate into mature myofibers. The surrounding extracellular matrix (ECM) together with inhibitory growth factors, such as transforming growth factor-beta (TGF-beta), also likely play an important role in growth control and muscle differentiation. Decorin, biglycan and betaglycan are proteoglycans that bind TGF-beta during skeletal muscle differentiation. In this paper, we show that the binding of TGF-beta to the receptors TGF-beta RI and-beta RII diminished in a satellite cell-derived cell line during differentiation, in spite of an increase expression of both receptors. In contrast, during the differentiation of decorin-null myoblasts (Den null), which lack decorin expression, the binding of TGF-beta to TGF-beta RI and -beta RII increased concomitantly with receptors levels. Both the addition and re-expression of decorin, in these myoblasts, diminished the binding of TGF-beta to its transducing receptors. Similar results were obtained when biglycan was added or over-expressed in Den null myoblasts. The binding of TGF-beta to TGF-beta RIII, alternatively known as betaglycan, was also augmented in Den null myoblasts and diminished by decorin, biglycan and betaglycan. These results suggest that decorin, biglycan and betaglycan compete for the binding of TGF-beta to its transducing receptors. Transfection studies with the TGF-beta-dependent promoter of the plasminogen activator inhibitor-1, coupled with luciferase, revealed that the addition of each proteoglycan diminished TGF-beta-dependent activity, for both TGF-beta 1 and -beta 2. The modulation of TGF-beta signaling by ECM proteoglycans diminishing the bio-availability of TGF-beta for its transducing receptors appears to be a feasible mechanism for the attenuation of this inhibitory growth factor during skeletal muscle formation. (c) 2006 Elsevier B.V./Intemational Society of Matrix Biology. All rights reserved.